The present invention generally relates to air conditioning and refrigeration systems and more particularly to an air conditioning system for use in conjunction with interior confined space, which exhibits significantly improved energy efficiency at low outdoor ambient temperatures.
Air conditioning systems comprising a compressor, a condenser, expansion valve(s), and an evaporator associated in a cyclical relationship, have two basic temperature variables placed upon them to which they must respond. One variable is the load placed on the evaporator which piece of equipment is located within a confined space which is to be cooled. The second variable placed upon the air conditioning system is the outdoor ambient temperature to which the condenser is subject. While virtually all air conditioning and refrigeration systems must respond to the same outdoor ambient temperature placed upon the condenser, the evaporator loads may vary drastically depending upon the intended use of the system. For example, refrigeration systems may be utilized for maintaining frozen-food cases in grocery stores wherein extremely low temperatures must be maintained, but a subtantially constant load is placed upon the evaporator. Another example concerns air conditioning loads placed upon systems designed to maintain the temperature within large buildings. Dramatic temperature differentials between one side of the building facing the sun and the opposing side in the shade cause very great variable loads to be placed upon the evaporator.
Another class of air conditioning systems involves designs structured to maintain specific rooms or sections interiorly located of a building at a substantially constant temperature and humidity. Such systems are required, for example, to maintain proper computer room environments. These systems must be capable of recourse to variable loads placed upon the evaporator to maintain substantially constant temperature and humidity conditions within the enclosed space. While the such loads as are witnessed within computer room environments generally are normally substantially constant, the system must be effectively responsive should variable load conditions be placed upon it.
Regardless of the particular air conditioning or refrigeration system under consideration, its location in regions which are subject to distinct seasonal temperature variations can strain its performance especially during winter months when the condenser is subject to low outdoor ambient temperatures. As the outdoor ambient temperature decreases, a corresponding decrease in the head pressure from the compressor occurs. As the head pressure decreases, an adequate pressure drop across the expansion valving becomes difficult to maintain. A variety of techniques have been proposed to efficiently utilize the cold outdoor temperature in providing additional cooling capacity for air conditioning systems. Prior proposals along these lines are described, for instance, in U.S. Pat. Nos. 2,252,300, 3,977,205, 3,952,533, 3,638,446, and 3,934,426. These proposals devise various expansion valve schemes for attempting to maintain an adequate pressure drop thereacross at lower head pressures caused, for example, by decreasing outdoor ambient temperatures. Other systems utilize by-pass pumping techniques for pumping liquid refrigerant directly through an evaporator when low outdoor ambient temperatures are encountered (U.S. Pat. Nos. 2,244,312 and 3,133,424).
These latter proposals are a small step forward, though substantially reduced energy consumption does not result from their implementation. As to the former proposals relating to multiple valving systems, decreasing outdoor ambient temperatures serve to provide increasing heat removal capabilities at the evaporator. This means that such systems eventually must be shut off when the desired room temperature has been reached due to the cold outdoor ambient temperatures impressed upon the condenser. Many of these systems, then, must have the compressors turned off and on very frequently during the winter months. As a consequence, the compressors necessarily will be subject to extreme wear and their life expectancy will be decreased accordingly.
Loads typically placed on such systems as are intended for computer room or refrigeration usage and the like tend to be comparatively static. When very wide load profiles are expected, however, the art has embarked upon a vastly different avenue for solution. Such avenue has included the use of variable speed, often inverter-driven, compressors as shown in U.S. Pat. Nos. 3,355,906, 4,257,238, 4,367,237, 3,449,992, and 3,499,297. With very wide load profiles, the air conditioning system must be able to rapidly change the cooling capacity which the evaporator possesses in the circuit. The variable speed compressor, usually a centrifugal compressor, has to a large degree provided the necessary flexibility in such systems.